Flow measuring devices are applied in industrial measurements technology for measuring volume flow, e.g. volume flow rate.
A measuring method current today rests on the magneto-hydrodynamic principle and is applied in conjunction with magneto-inductive flow measuring transducers. In such case, a medium, at least slightly electrically conductive, and whose volume flow is to be measured, is allowed to flow through a measuring tube, through which a magnetic field passes essentially perpendicularly to the measuring tube axis. Charge carriers moved perpendicularly to the magnetic field induce a voltage perpendicular to the direction of the flow of the medium. Such voltage can be tapped via appropriately arranged, measuring electrodes. The measuring electrodes are coupled for such purpose either capacitively or galvanically with the medium. The induced voltage is proportional to a flow velocity averaged over a cross section of the measuring tube and, thus, proportional also to volume flow.
Conventional flow measuring devices, especially magneto-inductive flow measuring devices, employ a measuring transducer, which includes a measuring tube inserted into an existing pipeline system. During measurement, medium flows through the measuring tube. To bring this about, process connections are provided on the measuring transducer, on both ends of the measuring tube. The process connections are in direct contact with the medium and are mounted directly on the measuring transducer.
Due to the high mechanical stability required for such measuring tubes, they are preferably composed of an external, especially metal, support tube of predeterminable strength and diameter. Internally, the support tube is covered with an electrically non-conductive, insulating material of predeterminable thickness, the so-called liner. There are, for example, magneto-inductive, flow measuring transducers, which involve a measuring tube having an inlet-side, first end and an outlet-side, second end. Such a measuring tube is, in turn, composed of a, most-often, non-ferromagnetic, support tube, as an outer jacket of the measuring tube, and a tubular liner of an insulating material, accommodated in a lumen of the support tube, for guiding a flowing medium isolated from the support tube. The liner serves for the chemical isolation of the support tube from the medium. In the case of support tubes of high electrical conductivity, especially in the case of metal support tubes, the liner serves, additionally, as electrical insulation between the support tube and the medium, in order to prevent a short-circuiting of the electrical field via the support tube. By an appropriate design of the support tube, there is thus obtained, a fitting of the strength of the measuring tube to the mechanical loads present in the given case of application; while, by means of the liner, a matching of the measuring tube to the demands, chemical, and, especially, hygienic, relevant for the given case of application can be realized. Applied often for manufacturing the liner are the injection-molding- or transfer-molding-processes. It is, however, also usual to insert a completely prefabricated liner, e.g. one of a thermoplastic or thermosetting, synthetic material, into the support tube.
Flawless measurement is only assured, when the medium and the measuring transducer lie at the same electrical potential. For this, preferably, both the medium and also the housing of the measuring transducer are provided with a ground connection. To the extent that the process connections are made of electrically conductive material, as a rule metal, the equalization of potential is accomplished directly via the process connections, which, in measurement operation, are in contact both with the medium and also with the measuring transducer. Then, there is no need for further potential-equalizing measures. The medium is immediately grounded via the grounding of the measuring transducer, for instance its housing.
There are, however, a large number of applications, in which process connections preferably of insulating material, especially plastic, are used. In this case, special measures must be introduced, in order to assure an equalizing of potential between medium and measuring transducer. The assignee of the invention described and claimed herein, offers today, for this purpose, for example in conjunction with its magneto-inductive flow measuring device, Promag H, a grounding ring, which is installed between the measuring transducer and the process connection and lies, in measurement operation, in contact both with the medium and also with the measuring transducer. The grounding ring effects the necessary equalizing of potential. FIG. 1 shows an example of this. Illustrated are a process connection 1 and a measuring transducer 3. Between process connection 1 and measuring transducer 3 is arranged a grounding ring 5. The grounding ring 5 is made of an electrically conductive material and is in direct contact with the medium during measurement operation. The medium flows through the grounding ring 5. Simultaneously, the grounding ring is, in its assembled state, in direct contact with the housing of the measuring transducer 3. Between the grounding ring 5 and the process connection 1 and between grounding ring 5 and the measuring transducer 3, there is, in each case, a seal 7, 9, in this instance, in each case, an O-ring.
The material of the grounding ring 5 is selected as a function of the material of the measuring electrodes of the magneto-inductive measuring transducer, in order to avoid electrochemical corrosion of the measuring electrodes. Moreover, of course, a material must be selected, which assures sufficient chemical and/or mechanical durability of the grounding ring 5 with respect to the medium. Depending on application, it can be necessary to apply relatively expensive, special materials.
The size of the grounding ring 5 depends on the nominal diameter of the particular process connection. Correspondingly, the amount of material needed for the grounding ring 5 depends on the nominal diameter. Depending on the size of the grounding ring 5 and the choice of material, this form of potential-equalization can be associated with considerable costs.